1. Field of the Invention
This invention relates to woven forming fabrics for papermaking machines.
2. Description of the Prior Art
In a Fourdrinier papermaking machine, paper stock, also called "furnish" or "stuff", is fed onto the top or outer surface of a travelling endless belt, which serves as the papermaking surface of the machine. The bottom or inner surface of the endless belt is supported on and driven by rolls associated with the machine. Fourdrinier belts, also known as Fourdrinier wires, forming media or forming fabrics, are commonly formed from a length of woven fabric with its ends joined together in a seam to provide an endless belt. Forming fabrics of this type generally comprise a plurality of spaced longitudinal warp threads and a plurality of spaced transverse weft threads which have been woven together on a suitable loom.
Initially, forming fabrics were woven wire structures made from materials such as phosphor bronze, bronze, stainless steel, brass or suitable combinations thereof. Recently in the papermaking field, it has been found that synthetic materials may be used in whole or in part to produce forming fabrics of superior quality. Nylon, a polyamide fiber, has been found to be suitable. Other examples of suitable materials are polyesters, such as Dacron, or acrylic fibers such as Orlon, dynel and Acrilan, or copolymers, such as saran. The warp and weft yarns of the forming fabric may be of the same or different constituent materials and/or constructions, and may be in the form of monofilament or multifilament yarns.
A very common weave used in making forming fabrics is referred to as the "four-harness satin" or "three-by-one-broken-twill". In this weave the warp yarns each pass over one and under three weft yarns to make a broken twill pattern of relatively short knuckles across the cloth.
One of the major problems caused by certain twill and satin weaves is a phenomenon called "pairing", which will be explained in detail hereinafter. Suffice it to say at this point, pairing is the misalignment of adjacent warp or weft yarns caused when two floats are in close proximity within a weave pattern. A float is a portion of a weft yarn that passes over two or more warp yarns or a portion of a warp yarn that passes over two or more weft yarns before interweaving. Pairing in a forming fabric results in drainage holes of varying sizes and affects overall drainage during the papermaking process. In addition, loose furnish fibers may penetrate through larger holes and build up on machine rolls. Further, the inconsistency of hole size in the forming fabric affects the overall smoothness of the paper produced during the papermaking process.
Since paper manufacturing machines are among the most delicately adjusted pieces of machinery used in any industry there is a great need to find a solution to the pairing problem. The subject invention provides that solution.